Investigate pubkey cache builds at skipped slots

[michaelsproul]

Copied from:

• Pubkey cache fix for skipped slots #7213

Skipped slots can import new validators, i.e. validators get added to the state when advancing to the epoch boundary (because of new finalization) but not added to the cache. Then when we try to verify a chain segment or a block in the next epoch from one of these validators, we fail due to the cache not being updated

I didn't actually managed to repro the bug I thought existed on that PR. I think it still exists though.